Large dynamoelectric machines employed in the utility industry use a long insulated conductor called a stator bar 1 (see FIG. 1-A) placed in its core. Construction of the stator bar 1 starts with a long, straight bar stock 2 (see FIG. 2-A) of precise rectangular cross-section, made of plurality of transposed insulated copper strands (not shown). Both ends of the bar stock 2 are then bent at sections A and B near the end to form end-arms AC and BD. The bar stock after bending is termed a bent-bar 3 (see FIG. 2-B) herein. The stator bar 1 in FIG. 1-A is then formed by insulating the bent-bar 3 by a high voltage-resistant insulation, termed ground wall insulation or bar insulation 4. The bar insulation 4 protects the core 10 against the large voltages of stator bar, which can range from 10 to 25 kV. The stator bar 1 hence contains a straight portion AB and bent end-arm portions AC and BD. When the stator bar 1 is inserted into the core 10, the straight portion AB protrudes beyond the end faces PQ of the core 10. The protruding straight portions PA and QB are called overhangs. The core 10 itself is made of lamination stack 11.
The bar insulation 4 thus forms a hollow rectangular sectioned shell around the bent-bar 3. Long ago, the bar insulation was made by wrapping a thin electrically insulative tape several times over a bent-bar 3 and curing it to form a solid bar. The thermal conductivity of the tape used to be about 0.3 w/mK, so machines used to be relatively large to keep it cool. In the past 20 years, electrically insulative tapes with thermally conductive fillers were developed, which increased the thermal conductivity to about 0.5 w/mK. A bar insulation with such higher thermal conductivity can reduce the stator temperature, beneficially increasing the life of the machine and its performance. In recent years, more innovative approaches to improving the thermal conductivity of bar insulation further are emerging, as summarized below.
Almost all prior-art approaches to improve the thermal conductivity of bar insulation employ thermally conductive fillers, and can be grouped into a multiphase tape 4a (FIG. 1-B), a multiphase extrusion 5 (FIG. 1-C) or a multiphase fabric. They use multiphased insulation, comprising a matrix or major phase of thermally insulative materials embedding minor phases of thermally conductive fillers. But admixture of multiple phases creates a large number of boundaries with innumerable discontinuities; it also suffers from potential trapped air spaces. The discontinuities and trapped air spaces obstruct heat flow, thereby reducing the net thermal conductivity. For example, the multiphase tape 4a may have one, two or more layers, each comprising a thermally insulative base layer with one phase of plastic tape to serve as a carrier, a second phase of fibrous weave to impart strength, a third phase of electrically insulative materials such as mica to provide electrical insulation, a fourth phase of thermally conductive fillers to provide thermal conductivity and a fifth phase of thermally insulative resin binder to bond all the phases. Several patents, e.g., U.S. Pat. No. 7,547,847 or 6,242,825 describe various layers, phases and materials used in multiphase tapes.
Even though prior-art bar insulation comprises more than half-dozen phases, the thermally conductivity of filler is the only phase that contributes to increase in thermal conductivity. All other phases, being thermally insulative, tend to reduce the thermal conductivity. The geometry of a filler greatly affects its thermal resistance. The fillers geometry can be long fibers, short whiskers (e.g., E-glass with 0.99 w/mK, Dacron glass with 0.4 W/mK), particulates (e.g., boron nitride with ˜120 w/mK, aluminum oxide with 25 w/mK etc.) or flakes. Fillers with fiber or whisker geometry are mostly thermally insulative because the round shape allows thermal contact only along a line, thus obstructing heat flow; they are usually encapsulated by a resin phase that is insulative and hence obstructs heat flow. Particulate fillers are microscopic, with the largest dimension less than 15 μm (600 μinch) but microscopic nano-fillers of size less than 0.2 μm (8 μinch), are recently being introduced per U.S. Pat. Nos. 7,875,347, 7,803,457. Heat flows easily inside flake shaped particulates because of favorable aspect ratio and high thermal conductivity, but outside the flakes surface, heat transfer is obstructed by the insulative resin phase that bonds them. The problem with all fillers is that they are encapsulated by a resin binder which is mostly thermally insulative, hence overall thermal conductivity is not increased.
U.S. Pat. No. 7,655,868 describes an alternative embodiment that uses a tape made of thermally conductive fabric. The fabric in this invention is made by weaving a thermally conductive ceramic fiber phase in one or both directions. But a high thermal conductivity of fiber phase does not necessarily increase the thermal conductivity of bar insulation because, even though the heat flows easily through them, it encounters great resistance when trying to cross a boundary. These ceramic fibers have round cross section that contact only at a point or along a line, and this narrow path of transmission obstructs heat flow. Besides introduction of the ceramic fibers does not solve the fundamental problems of air voids and resin encapsulation that reduce the thermal conductivity.
Recognizing the fundamental limitations of tapes, extrusions as alternatives to tapes to increase thermal conductivity were also examined recently. The extrusion can be an in-situ or a pre-extruded member. An in-situ extruded insulation was described in U.S. Pat. Nos. 5,650,031 and 5,710,475; the method uses a moving extrusion head to deposit thermoplastic resin in-situ over a stationary bent-bar 3. But it is very difficult for an extrusion head to follow the complex 3-D shape of bent-bar 3. The technical difficulties of precisely depositing resin over complex 3-D shape of end-arms AC and BD are so challenging that currently stator bars are not made by this in-situ extrusion method. Alternately, pre-extruded members for bar insulation were also discussed in U.S. Pat. No. 7,832,081. FIG. 1-C shows one such embodiment in which the bar insulation comprises a pre-extruded insulating member 5 having a rectangular shape that defines a central cavity 9. The extruded member has a slit 7 along the entire length of the bent-bar 3. The slit 7 is made of pair of opposite edges 8. Bent-bar 3 is slipped into the cavity 9 and the slit 7 is closed by plastic welding. But an extrusion insulation 5 with a slit 7 has several disadvantages. Long plastic welds that are used to close the long slit could contain electrical defects at some points over a long span. Besides, in vibratory high temperature environment, the weld expands and contracts several million times, so it may develop a crack which could propagate and cause catastrophic degradation of the insulation. Besides, manufacturing a slitted extrusion that precisely conforms to the complex 3-D shape is technically challenging. Further, extrusions rely on fillers to improve thermal conductivity, but it is well known that fillers do not greatly enhance thermal conductivity of the bar insulation. In view of these deficiencies, there still remains a need for bar insulation with enhanced thermal conductivity.